The present disclosure relates to an electronic circuit, more particularly, to a fast start-up circuit for a bandgap reference voltage generator.
A bandgap reference voltage generator can provide a voltage reference that is independent of the voltage supply and temperature. A bandgap reference voltage generator is commonly used in mixed-signal designs that include analog blocks. A bandgap reference voltage generator is similar to a voltage regulator and a current reference. A bandgap reference voltage generator typically includes two operation conditions for the bandgap; a power-down point and a start-up point. The power-down point can provide bias to the gates of PMOS transistors at the voltage of the high voltage supply (VDD) to prevent current flowing through the bandgap reference voltage circuit. The bandgap output voltage is kept near the bandgap voltage of the semiconductor substrate. The bandgap voltage for Silicon-based substrate is about 1.25V. At the start-up point, the PMOS transistors are biased in saturation region. A DC current flows through the bandgap reference voltage circuit. The bandgap output voltage is around 1.25V, which is the bandgap voltage of silicon.
Some conventional bandgap reference voltage generators can provide proper start-up operations. However, the conventional bandgap reference voltage generators usually include several drawbacks. Some conventional bandgap reference voltage generators cannot provide a fast start-up. They take quite a long time to start from a power-down state. Some conventional bandgap reference voltage generators draw static DC current after the bandgap voltage reference circuit becomes stable.
Referring to FIG. 1, a conventional bandgap voltage reference circuit 100 includes an operational amplifier (op-amp) 110, bipolar transistors 114 and 115 that are connected as diodes, resistors 111-113, and a biased PMOS transistor 108. The resistors 111-113 respectively have resistance Ra, Rb, and Rc. VBG is the bandgap voltage output.
A relation between current I and voltage Vf in a diode can be expressed as
                                                                        I                =                                ⁢                                                      I                    S                                    ×                                      (                                          ⅇ                                                                        q                          ⁡                                                      (                                                                                          V                                f                                                            /                              kT                                                        )                                                                          -                        1                                                              )                                                                                                                          ≅                                ⁢                                                      I                    S                                    ×                                      (                                          ⅇ                                              q                        ⁡                                                  (                                                                                    V                              f                                                        /                            kT                                                    )                                                                                      ⁢                                                                                                                                                    ⁢                                  ⁢                              for            ⁢                                                  ⁢                          V              f                                ⪢                      kT            /            q                                              (        1        )                                          V          f                =                              V            T                    ×                      ln            ⁡                          (                              I                /                                  I                  S                                            )                                                          (        2        )            where k is Boltzmann constant (1.38×10−23 J/K), IS is a constant describing the transfer characteristic of the transistor in the forward-active region, q is the electronic charge (1.6×10−19 C), and the thermal voltage VT is defined by VT=kT/q. Two inputs for the operational amplifier 110 are connected to nodes 150 and 151. The operational amplifier 110 is designed to keep the voltages at the nodes 150 and 151 the same. Defining Vf1 and Vf2 respectively as the diode voltages of the bipolar transistors 114, 115 that are connected as diodes, the difference between Vf1 and Vf2 isdVf=Vf1−Vf2=VT ln(N Rb/Rc)  (3)where N is the geometric ratio of bipolar transistor 115 to 114. N is normally equal to 4. The bandgap output voltage VBG then becomes
                                                        VBG              =                                                Vf                  ⁢                                                                          ⁢                  1                                +                                                      (                                          Rb                      /                      Rc                                        )                                    ⁢                  dVf                                                                                                        =                                                Vf                  ⁢                                                                          ⁢                  1                                +                                                      (                                          Rb                      /                      Rc                                        )                                    ⁢                                      V                    T                                    ⁢                                      ln                    ⁡                                          (                                              N                        ⁢                                                                                                  ⁢                                                  Rb                          /                          Rc                                                                    )                                                                                                                              (        4        )            Vf1 has a negative temperature coefficient of −2 mV/° C., whereas VT has a positive temperature coefficient of 0.086 mV/° C. VBG is determined by the resistance ratio, being little influenced by the absolute value of the resistance. By varying N, Rb, Rc, the bandgap voltage VBG can be tuned to be around the bandgap energy of the semiconductor substrate. For a silicon-based integrated circuit, VBG can thus be controlled to be about 1.25V. Further, the temperature dependence of VBG can be negligibly small. In addition, VBG does not depend on supply voltage as long as the power supply is not lower than VBG.
A drawback associated with the bandgap voltage reference circuit 100 is that the bandgap voltage reference circuit 100 cannot properly start up a circuit during a power down. During power up from power down state, the gate bias of PMOS transistor 108 is undefined and may be biased at VDD due to the coupling effect from VDD through its gate to source capacitance. Also, as the gate bias of PMOS transistor 108 is undefined, the start-up time is not controllable, which can lead to an unpredictable start-up operation.
FIG. 2 shows another conventional bandgap voltage reference circuit 200 having a start-up circuit 220. The start-up circuit 220 includes a PMOS transistor 206, a self-biased inverter (with its input connected to its output) consisting of a PMOS transistor 202 and an NMOS transistor 203, and an NMOS transistor 204. The gate of the PMOS transistor 206 is connected with the gates of the PMOS transistor 202 and an NMOS transistor 203 at a node 205. Other components of the bandgap voltage reference circuit 200 are similar to bandgap voltage reference circuit 100.
During power up, the node 205 is biased at (VDD−VTP+VTN)/2, where VTP is the PMOS threshold voltage and VTN is the NMOS threshold voltage. During power-up, the PMOS transistor 206 turns on the bias PMOS transistor 108 to allow current can flow through the bias PMOS transistor 108 to kick-start the bandgap voltage reference circuit 200. The gate voltage of the bias PMOS transistor 108 gradually rises up to reach a voltage at which it shuts off the PMOS transistor 206, thus disconnecting the start-up circuit 220. Even after the PMOS transistor 206 is shut off, however, a DC current continues to flow through the PMOS transistors 201 and 202 and NMOS transistors 203 and 204. The power consumption caused by the DC current after the circuit start-up is a significant drawback in the bandgap voltage reference circuit 200, which makes it unsuitable for portable devices that require low power consumption.
Furthermore, a bandgap voltage reference circuits 100 and 200 can suffer from slow start-ups. The transistors in the bandgap voltage reference circuits 100 and 200 are typically selected to be very small to minimize DC current consumption. Smaller transistors, however, have smaller trans-conductance, and typically take long time to charge and thus have long start-up times.
There is therefore a need for a bandgap reference voltage circuit that can provide a correct starting point for the bandgap reference voltage circuit, can quickly start from a power-down state, and has no or low power consumption after the start-up.